Amazing link.
Our model for barter: Barter is anything liquid (in the economic sense) that you, at this moment, want to exchange for another good.
So: I buy three cans of gas from Beansy (1 barter). I go to the market and use some extra veggies I have to buy a new 9mm. (1 barter). On the sheet, those are the same barter. Commodities are not mechanically tracked. The veggies didn't "exist" until I went to buy the gun. The fact that I bought gas and lost food are cues for the GM to possibly make me buy food later, but that I don't need to buy gas for a while. Simple.